Density Functional Theory Provides Insights into β-SnSe Monolayers as a Highly Sensitive and Recoverable Ozone Sensing Material

This study explores the potential of β-SnSe monolayers as a promising material for ozone (O3) sensing using density functional theory (DFT) combined with the non-equilibrium Green’s function (NEGF) method. The adsorption characteristics of O3 molecules on the β-SnSe monolayer surface were thoroughly investigated, including adsorption energy, band structure, density of states (DOSs), differential charge density, and Bader charge analysis. Post-adsorption, hybridization energy levels were introduced into the system, leading to a reduced band gap and increased electrical conductivity. A robust charge exchange between O3 and the β-SnSe monolayer was observed, indicative of chemisorption. Recovery time calculations also revealed that the β-SnSe monolayer could be reused after O3 adsorption. The sensitivity of the β-SnSe monolayer to O3 was quantitatively evaluated through current-voltage characteristic simulations, revealing an extraordinary sensitivity of 1817.57% at a bias voltage of 1.2 V. This sensitivity surpasses that of other two-dimensional materials such as graphene oxide. This comprehensive investigation demonstrates the exceptional potential of β-SnSe monolayers as a highly sensitive, recoverable, and environmentally friendly O3 sensing material.


Introduction
Ozone (O 3 ), as a trace gas in the atmosphere, possesses a strong oxidizing capacity that can irritate the respiratory tract and eyes, especially affecting children, the elderly, and those with respiratory diseases.When present above certain levels, ozone can adversely impact physical and mental health.Monitoring ozone levels is crucial for maintaining its relative stability in the atmosphere and ensuring air quality.
Currently, ozone monitoring primarily relies on two methods: spectrophotometry and chemiluminescence.The former utilizes ultraviolet characteristics to measure ozone concentration based on absorption and wavelength in a medium, while the latter determines concentration based on the intensity of emitted light.However, both methods require laboratory settings, are time-consuming, and incur high costs, making continuous real-time monitoring of ozone levels challenging.
Nanostructured materials have emerged as promising candidates for ozone sensing due to their inherent advantages.Their high surface-to-volume ratio facilitates enhanced Micromachines 2024, 15, 960 2 of 11 gas-solid interactions, promoting efficient ozone adsorption and subsequent sensing mechanisms.One-dimensional nanotubes, exemplified by pristine single-walled aluminum nitride nanotubes [1], and zero-dimensional nanocages, including Ni-doped Zn 12 O 12 [2], B 12 N 12 [3], BN fullerene-like structures [4], AlB 11 N 12 , GaB 11 N 12 , and Al 12 N 12 [5], have shown particular promise for ozone detection.These nanostructures possess hollow interiors and abundant unsaturated surface atoms, both of which contribute to their ozone sensing capabilities.
In addition to these burgeoning two-dimensional materials, tin-based materials have garnered significant attention for their potential in ozone sensing applications.Boron doping has been shown to enhance charge transfer from stanene to ozone molecules, positioning it as a promising candidate for the development of sensitive ozone sensors [12].Furthermore, the formation of heterojunctions, particularly by integrating stanene with materials like TiO 2 , has been investigated to exploit synergistic effects that further amplify ozone sensing performance [13].These advancements underscore the innovative strides being made in the realm of material science for environmental monitoring.Furthermore, SnS has been identified as exhibiting a high degree of efficacy in interacting with ozone, positioning it as a promising candidate for chemical sensing in ozone detection methodologies [14].
β-SnSe (hereafter referred to as SnSe), a layered transition metal chalcogenide with a graphene-analogous structure, has attracted considerable interest owing to its extraordinary attributes and diverse potential applications.At elevated temperatures, SnSe exhibits an exceptionally low lattice thermal conductivity, rendering it a promising material for thermoelectric energy conversion [15][16][17].Notably, SnSe possesses desirable characteristics, including chemical stability, non-toxicity, and abundant terrestrial reserves, making it suitable for large-scale applications and sustainable development.SnSe possesses a tunable band gap and superior light absorption properties, particularly in its monolayer form, where it demonstrates significant photoresponsiveness and exceptional photoelectric performance [18].In terms of gas sensing, SnSe has demonstrated strong chemical adsorption towards various gas molecules, such as methanol (CH 3 OH), oxygen (O 2 ), nitrogen dioxide (NO 2 ), and sulfur dioxide (SO 2 ), making it a compelling candidate for gas detection applications [19].
In this study, the potential performance of monolayer SnSe as an O 3 sensing material was systematically investigated using density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) method.Theoretical calculations provided an in-depth analysis of the adsorption characteristics of O 3 molecules on the surface of monolayer SnSe, involving adsorption energy, band structure, density of states (DOSs), differential charge density, and Bader charge analysis.These calculations revealed the interaction mechanism and electron transfer process between the monolayer SnSe and O 3 molecules, elucidating its fundamental sensing properties as an O 3 gas detector.Additionally, this work specifically considered the recoverability of monolayer SnSe in practical applications, evaluating the recovery time after O 3 adsorption through computational assessments.Furthermore, the response sensitivity of monolayer SnSe to O 3 gas was quantitatively explored based on current-voltage (I-V) characteristic simulations.This research demonstrates the immense potential of monolayer SnSe as a highly sensitive, recoverable, and environmentally friendly O 3 sensing material.

Materials and Methods
First-principle calculations were performed using the Vienna Ab initio Simulation Package (VASP) 5.4 within the framework of DFT [20,21].The exchange-correlation energy was treated with the generalized gradient approximation (GGA) in the form of the Perdew-Burke-Ernzerhof (PBE) functional [22].To accurately account for the potential weak interactions, such as van der Waals forces, between the molecules and the substrate, the DFT-D3 correction scheme was further employed [23].Spin polarization effects were also considered in the calculations.
Specifically, a 4 × 4 supercell system comprising 64 atoms was constructed to simulate the two-dimensional structure of an SnSe monolayer.An energy cutoff of 400 eV was set to ensure calculation accuracy.During the structural optimization, the maximum force on each atom was required to be lower than 0.02 eV/Å, and the total energy convergence criterion was also set to 10 −5 eV to ensure sufficient accuracy in the structural optimization.Considering the effects of periodic boundary conditions, a 20 Å vacuum layer was established in the perpendicular direction to prevent interactions between adjacent periods.For Brillouin zone sampling, a Monkhorst-Pack grid of 4 × 4 × 1 was utilized.
The adsorption energy (E ad ) of the O 3 molecule on the SnSe monolayer surface was calculated using the following equation: where E total is the total adsorption energy of the optimized systems after O 3 adsorption on the SnSe monolayer, E SnSe is the energy of the optimized pristine SnSe monolayer, and E O 3 denotes the energy of the O 3 molecule involved.The current-voltage characteristics (I-V curves) were conducted using the tranSiesta module within the Siesta 4.1.5software package, in conjunction with NEGF theory [24].Utilizing the Landauer-Büttiker formula [25], the electrical currents across the material under varying voltages were determined: Here, I signify the electric current traversing the contact electrode at the bias voltage (V b ).The terms µ L and µ R represent the electrochemical potentials of the left and right electrodes, respectively.T(E, V b ) denotes the transmission coefficient at voltage V b and energy E. The f(E − µ L ) accounts for the Fermi-Dirac distribution function of the left and right electrodes at energy E.
Additionally, the VESTA 3.90.0asoftware was used to analyze the differential charge density distribution of the system before and after adsorption, as well as the optimized geometric configuration [26].Finally, the vaspkit 1.3.3tool was employed for effective post-processing and analysis of the raw computational data [27].

Structures
As depicted in Figure 1, the SnSe monolayer comprises two facets: the tin (Sn) side and the selenium (Se) side.On the Sn side, the O 3 molecule may occupy four potential sites: atop the Sn atom (TSn1), atop the Se atom (Tse1), on the hollow site (H1), or on the bridge site (B1) between Sn and Se bonds.Similarly, on the Se side, the O 3 molecule may be located atop the Sn atom (Tsn2), atop the Se atom (Tse2), on the hollow site (H2), or on the bridge site (B2) between Sn and Se bonds.Upon adsorption to the surface, the O 3 molecule may approach in various orientations: with the lateral oxygen atoms near the surface and the central oxygen atom farther away, with a perpendicular arrangement to the SnSe surface; with the central oxygen atom near the surface and the lateral oxygen atoms farther away, with a perpendicular arrangement to the SnSe surface; or with the plane formed by the three oxygen atoms parallel to the SnSe surface.Energy calculations for different configurations indicate that the most stable and energetically favorable configuration is where the O 3 molecule bridges between the Sn and Se bonds, with the lateral oxygen atoms approaching the surface and the central oxygen atom remaining farther away, forming a perpendicular orientation to the Sn side of the SnSe surface.
different configurations indicate that the most stable and energetically favorable configuration is where the O3 molecule bridges between the Sn and Se bonds, with the lateral oxygen atoms approaching the surface and the central oxygen atom remaining farther away, forming a perpendicular orientation to the Sn side of the SnSe surface.As plotted in Figure 2a, the O3 molecule induces significant reconstruction of the SnSe monolayer surface during the adsorption process.Before adsorption, the Sn-Se bond length is approximately 2.744 Å, with a bond angle of 90.741°.However, upon O3 adsorption, the Sn atom beneath the adsorption site experiences a strong attractive force from the oxygen atoms, causing it to move closer to the oxygen atoms.This results in a contraction of the Sn-Se bond length to 2.657 Å, while the bond length between the Sn atom and the adjacent Se atoms elongates to 2.769 Å.Consequently, the Sn-Se-Se bond angle expands to 97.417°, reflecting the substantial impact of O3 adsorption on the SnSe surface structure.In the adsorption configuration, the distance between the closest oxygen atom of the O3 molecule and the Sn surface is 2.251 Å, suggesting the possibility of a strong adsorption interaction between Sn and the O3 molecule.different configurations indicate that the most stable and energetically favorable configuration is where the O3 molecule bridges between the Sn and Se bonds, with the lateral oxygen atoms approaching the surface and the central oxygen atom remaining farther away, forming a perpendicular orientation to the Sn side of the SnSe surface.As plotted in Figure 2a, the O3 molecule induces significant reconstruction of the SnSe monolayer surface during the adsorption process.Before adsorption, the Sn-Se bond length is approximately 2.744 Å, with a bond angle of 90.741°.However, upon O3 adsorption, the Sn atom beneath the adsorption site experiences a strong attractive force from the oxygen atoms, causing it to move closer to the oxygen atoms.This results in a contraction of the Sn-Se bond length to 2.657 Å, while the bond length between the Sn atom and the adjacent Se atoms elongates to 2.769 Å.Consequently, the Sn-Se-Se bond angle expands to 97.417°, reflecting the substantial impact of O3 adsorption on the SnSe surface structure.In the adsorption configuration, the distance between the closest oxygen atom of the O3 molecule and the Sn surface is 2.251 Å, suggesting the possibility of a strong adsorption interaction between Sn and the O3 molecule.calculated adsorption energy of the O 3 molecule on the SnSe monolayer is −1.826 eV.This negative value clearly indicates that the O 3 molecule adsorption on the SnSe surface is a thermodynamically spontaneous process that can proceed without the input of additional external energy.In other words, SnSe exhibits a strong affinity and easy adsorption for O 3 molecules and can effectively capture O 3 molecules at room temperature.Therefore, it is a promising candidate material for room-temperature O 3 gas sensors.

Electronic Properties
In the in-depth exploration of the intrinsic mechanism behind the O 3 gas sensing performance of monolayer SnSe, computational analysis was conducted on the electronic structure and band characteristics of monolayer SnSe before and after O 3 adsorption.The Fermi level is adjusted to the zero point, and the band structure near the Fermi level, which is crucial for semiconductor properties, is specifically presented within the energy range of −4 eV to 4 eV.
The pristine SnSe monolayer is calculated to exhibit a theoretical band gap of 2.228 eV, as illustrated in Figure 3a, which is in close agreement with the previously reported theoretical value of 2.22 eV [16].Upon O 3 molecule adsorption on the SnSe monolayer surface, its electronic structure undergoes significant reconstruction.The adsorption process introduces new impurity energy levels near the Fermi level, which originate from the valence band maximum (VBM) and conduction band minimum (CBM) of the SnSe monolayer.These impurity energy levels directly influence the original band structure of SnSe, as seen in Figure 3b, leading to a remarkable decrease in the band gap to 0.790 eV.
The calculated adsorption energy of the O3 molecule on the SnSe monolayer is −1.826 eV.This negative value clearly indicates that the O3 molecule adsorption on the SnSe surface is a thermodynamically spontaneous process that can proceed without the input of additional external energy.In other words, SnSe exhibits a strong affinity and easy adsorption for O3 molecules and can effectively capture O3 molecules at room temperature.Therefore, it is a promising candidate material for room-temperature O3 gas sensors.

Electronic Properties
In the in-depth exploration of the intrinsic mechanism behind the O3 gas sensing performance of monolayer SnSe, computational analysis was conducted on the electronic structure and band characteristics of monolayer SnSe before and after O3 adsorption.The Fermi level is adjusted to the zero point, and the band structure near the Fermi level, which is crucial for semiconductor properties, is specifically presented within the energy range of −4 eV to 4 eV.
The pristine SnSe monolayer is calculated to exhibit a theoretical band gap of 2.228 eV, as illustrated in Figure 3a, which is in close agreement with the previously reported theoretical value of 2.22 eV [16].Upon O3 molecule adsorption on the SnSe monolayer surface, its electronic structure undergoes significant reconstruction.The adsorption process introduces new impurity energy levels near the Fermi level, which originate from the valence band maximum (VBM) and conduction band minimum (CBM) of the SnSe monolayer.These impurity energy levels directly influence the original band structure of SnSe, as seen in Figure 3b, leading to a remarkable decrease in the band gap to 0.790 eV.This significant band gap narrowing implies that the conductivity of the SnSe monolayer is greatly enhanced after O3 adsorption.The newly formed impurity energy levels allow electrons in the valence band to jump to the conduction band at a lower energy threshold.Compared to the state before O3 adsorption, the activation energy required for electron migration is significantly reduced.Such changes in the electronic structure greatly improve the sensitive response efficiency of the SnSe monolayer to external stimuli as an O3 gas sensor.
Figure 4 delineates the total density of states (TDOSs) and partial density of states (PDOSs), where the adsorption of O3 on SnSe introduces hybridization energy levels flanking the Fermi level.The TDOSs analysis reveals a pronounced symmetry in the DOS curves for both spin-up and spin-down configurations, indicating the non-magnetic nature of the adsorption structure.Notably, significant peaks at energy values of −0.085 eV and 0.697 eV are observed, predominantly originating from the p orbitals of oxygen atoms.This significant band gap narrowing implies that the conductivity of the SnSe monolayer is greatly enhanced after O 3 adsorption.The newly formed impurity energy levels allow electrons in the valence band to jump to the conduction band at a lower energy threshold.Compared to the state before O 3 adsorption, the activation energy required for electron migration is significantly reduced.Such changes in the electronic structure greatly improve the sensitive response efficiency of the SnSe monolayer to external stimuli as an O 3 gas sensor.
Figure 4 delineates the total density of states (TDOSs) and partial density of states (PDOSs), where the adsorption of O 3 on SnSe introduces hybridization energy levels flanking the Fermi level.The TDOSs analysis reveals a pronounced symmetry in the DOS curves for both spin-up and spin-down configurations, indicating the non-magnetic nature of the adsorption structure.Notably, significant peaks at energy values of −0.085 eV and 0.697 eV are observed, predominantly originating from the p orbitals of oxygen atoms.At −0.085 eV, the oxygen-dominated absorption peak triggers peak responses in the s and p orbitals of Sn atoms and the p orbitals of Se atoms at the same energy level, unveiling a robust orbital hybridization indicative of changes in electron cloud distribution and energy level rearrangement during adsorption.Similarly, at 0.697 eV, oxygen atoms induce corresponding peaks in the s orbitals of Sn atoms and the p orbitals of Se atoms.These enhancements in electron density at specific energy levels directly affect the material's band structure, particularly introducing impurity levels near the top of the valence band and altering the electronic transport characteristics near the bottom of the conduction band, thereby modifying the material's electrical conductivity and reactive properties post-adsorption.
To elucidate the bonding characteristics of adsorbed atoms on the SnSe monolayer surface, the charge density differential was computed using the following equation: where Δρ represents the charge density differential, ρSnSe+O3 is the charge density of the adsorption system, ρSnSe is the charge density of the SnSe monolayer surface, and ρO3 is the charge density of the adsorbed O3 molecule.Figure 2b presents three-dimensional isosurface plots of the differential charge density, where yellow regions represent electron accumulation and blue regions represent electron depletion.As seen in Figure 2b, when the O3 molecule is adsorbed on the SnSe monolayer surface, a significant electron depletion region appears on the Sn atom side opposite to the O atom, while a large amount of electron accumulation occurs around the O atom.This indicates that the O atom gains electrons from the Sn atom.Additionally, Figure 2b also reveals that the surface of the three Se atoms bonded to the O3 molecule also exhibits electron depletion regions, further confirming the transfer of electrons from the SnSe monolayer to the O3 molecule during the adsorption process.
Bader charge analysis reveals that the O3 molecule gains a net charge of 0.881 electron units, while the bonded Sn atom loses 1.200 electron units.This result provides strong At −0.085 eV, the oxygen-dominated absorption peak triggers peak responses in the s and p orbitals of Sn atoms and the p orbitals of Se atoms at the same energy level, unveiling a robust orbital hybridization indicative of changes in electron cloud distribution and energy level rearrangement during adsorption.Similarly, at 0.697 eV, oxygen atoms induce corresponding peaks in the s orbitals of Sn atoms and the p orbitals of Se atoms.These enhancements in electron density at specific energy levels directly affect the material's band structure, particularly introducing impurity levels near the top of the valence band and altering the electronic transport characteristics near the bottom of the conduction band, thereby modifying the material's electrical conductivity and reactive properties post-adsorption.
To elucidate the bonding characteristics of adsorbed atoms on the SnSe monolayer surface, the charge density differential was computed using the following equation: where ∆ρ represents the charge density differential, ρ SnSe+O 3 is the charge density of the adsorption system, ρ SnSe is the charge density of the SnSe monolayer surface, and ρ O 3 is the charge density of the adsorbed O 3 molecule.Figure 2b presents three-dimensional isosurface plots of the differential charge density, where yellow regions represent electron accumulation and blue regions represent electron depletion.As seen in Figure 2b, when the O 3 molecule is adsorbed on the SnSe monolayer surface, a significant electron depletion region appears on the Sn atom side opposite to the O atom, while a large amount of electron accumulation occurs around the O atom.This indicates that the O atom gains electrons from the Sn atom.Additionally, Figure 2b also reveals that the surface of the three Se atoms bonded to the O 3 molecule also exhibits electron depletion regions, further confirming the transfer of electrons from the SnSe monolayer to the O 3 molecule during the adsorption process.
Bader charge analysis reveals that the O 3 molecule gains a net charge of 0.881 electron units, while the bonded Sn atom loses 1.200 electron units.This result provides strong evidence for a significant charge exchange phenomenon occurring between the O 3 molecule and the surface, suggesting the high adsorption stability of the O 3 molecule on the SnSe surface.

Recovery Time
Recovery time is a critical metric for evaluating the reversibility of sensors, referring to the time required to desorb target gas molecules from the surface of the sensing material.Recovery time τ is typically inversely related to adsorption energy and can be estimated through transition state theory: where E ad is the adsorption energy, k B is the Boltzmann constant, and T is the temperature.ω is the attempt frequency, assumed to be 10 13 s −1 [28].Figure 5 illustrates how temperature affects the recovery time of the SnSe monolayer following O 3 molecule adsorption.The recovery time for the SnSe monolayer following O 3 adsorption can reach 7.61 × 10 17 s at an ambient temperature of 298 K.This finding underscores the exceptional O 3 adsorption potential of SnSe even at room temperature.The O 3 molecules' desorption time, however, drastically decreases to 244.91 s as the temperature steadily rises to 598 K.At a temperature of 698 K, the recovery time is observed to be 1.53 s.This observation implies that at higher temperatures, the adsorption of O 3 on the SnSe monolayer becomes more reversible, enabling a rapid recovery to the initial state and demonstrating excellent dynamic response performance.
evidence for a significant charge exchange phenomenon occurring between the O3 molecule and the SnSe surface, suggesting the high adsorption stability of the O3 molecule on the SnSe surface.

Recovery Time
Recovery time is a critical metric for evaluating the reversibility of sensors, referring to the time required to desorb target gas molecules from the surface of the sensing material.Recovery time τ is typically inversely related to adsorption energy and can be estimated through transition state theory: where Ead is the adsorption energy, kB is the Boltzmann constant, and T is the temperature.ω is the attempt frequency, assumed to be 10 13 s −1 [28].Figure 5 illustrates how temperature affects the recovery time of the SnSe monolayer following O3 molecule adsorption.The recovery time for the SnSe monolayer following O3 adsorption can reach 7.61 × 10 17 s at an ambient temperature of 298 K.This finding underscores the exceptional O3 adsorption potential of SnSe even at room temperature.The O3 molecules' desorption time, however, drastically decreases to 244.91 s as the temperature steadily rises to 598 K.At a temperature of 698 K, the recovery time is observed to be 1.53 s.This observation implies that at higher temperatures, the adsorption of O3 on the SnSe monolayer becomes more reversible, enabling a rapid recovery to the initial state and demonstrating excellent dynamic response performance.

Sensitivity
To gain insight into the sensing capabilities of SnSe towards ozone, the sensitivity (S) of SnSe is calculated using the following equation: where I and I0 are the currents across the scattering region when adsorbed with O3 and in their pristine condition, respectively.An examination of the current-voltage (I-V) characteristics appears in Figure 6, which clarifies that O3 adsorption has very little effect on the semiconductor surface current at lower bias voltage ranges (≤0.6 V).This finding points to a slight difference in current values between the O3 adsorption and non-adsorption states.This suggests that at

Sensitivity
To gain insight into the sensing capabilities of SnSe towards ozone, the sensitivity (S) of SnSe is calculated using the following equation: where I and I 0 are the currents across the scattering region when adsorbed with O 3 and in their pristine condition, respectively.An examination of the current-voltage (I-V) characteristics appears in Figure 6, which clarifies that O 3 adsorption has very little effect on the semiconductor surface current at lower bias voltage ranges (≤0.6 V).This finding points to a slight difference in current values between the O 3 adsorption and non-adsorption states.This suggests that at low voltage driving, O 3 adsorption does not significantly alter the carrier transport characteristics of the semiconductor interface.Substituting the calculated values into Equation ( 5) yields the sensitivity-voltage curve.As depicted in Figure 7, the sensitivity-voltage curve of SnSe for O 3 at room temperature indicates that within the 0-0.6 V range, the sensitivity of SnSe to O 3 detection remains relatively low and is consistently below 100%.low voltage driving, O3 adsorption does not significantly alter the carrier transport characteristics of the semiconductor interface.Substituting the calculated current values into Equation ( 5) yields the sensitivity-voltage curve.As depicted in Figure 7, the sensitivityvoltage curve of SnSe for O3 at room temperature indicates that within the 0-0.6 V range, the sensitivity of SnSe to O3 detection remains relatively low and is consistently below 100%.However, within the bias voltage range of 0.7 to 1.2 V, the influence of adsorbed O3 molecules on the surface current becomes more pronounced, resulting in SnSe exhibiting a sensitivity to O3 exceeding 200%.At a bias voltage of 1.2 V, the current response of the O3-adsorbed surface exhibits a marked enhancement, reaching a value of 2.54 × 10 −6 µA.This represents an approximately one-order-of-magnitude increase compared to the current measured for the unadsorbed surface (1.33 × 10 −7 µA), indicating a high degree of sensitivity.Consequently, the sensor system exhibits a remarkable sensitivity of 1817.57% at a working voltage of 1.2 V.This strongly proves that under high voltage excitation, the interaction between O3 and the semiconductor interface will greatly promote charge transfer and induce significant resistance changes, thus enabling the SnSe monolayer to have a highly sensitive detection capability for O3 molecules.low voltage driving, O3 adsorption does not significantly alter the carrier transport characteristics of the semiconductor interface.Substituting the calculated current values into Equation ( 5) yields the sensitivity-voltage curve.As depicted in Figure 7, the sensitivityvoltage curve of SnSe for O3 at room temperature indicates that within the 0-0.6 V range, the sensitivity of SnSe to O3 detection remains relatively low and is consistently below 100%.However, within the bias voltage range of 0.7 to 1.2 V, the influence of adsorbed O3 molecules on the surface current becomes more pronounced, resulting in SnSe exhibiting a sensitivity to O3 exceeding 200%.At a bias voltage of 1.2 V, the current response of the O3-adsorbed surface exhibits a marked enhancement, reaching a value of 2.54 × 10 −6 µA.This represents an approximately one-order-of-magnitude increase compared to the current measured for the unadsorbed surface (1.33 × 10 −7 µA), indicating a high degree of sensitivity.Consequently, the sensor system exhibits a remarkable sensitivity of 1817.57% at a working voltage of 1.2 V.This strongly proves that under high voltage excitation, the interaction between O3 and the semiconductor interface will greatly promote charge transfer and induce significant resistance changes, thus enabling the SnSe monolayer to have a highly sensitive detection capability for O3 molecules.However, within the bias voltage range of 0.7 to 1.2 V, the influence of adsorbed O 3 molecules on the surface current becomes more pronounced, resulting in SnSe exhibiting a sensitivity to O 3 exceeding 200%.At a bias voltage of 1.2 V, the current response of the O 3 -adsorbed surface exhibits a marked enhancement, reaching a value of 2.54 × 10 −6 µA.This represents an approximately one-order-of-magnitude increase compared to the current measured for the unadsorbed surface (1.33 × 10 −7 µA), indicating a high degree of sensitivity.Consequently, the sensor system exhibits a remarkable sensitivity of 1817.57% at a working voltage of 1.2 V.This strongly proves that under high voltage excitation, the interaction between O 3 and the semiconductor interface will greatly promote charge transfer and induce significant resistance changes, thus enabling the SnSe monolayer to have a highly sensitive detection capability for O 3 molecules.
To comprehensively assess the detection capability of SnSe for O 3 across different temperatures, the impact of temperature variations on the current-voltage (I-V) characteristics and sensitivity was investigated.The I-V curves at 298 K and 698 K are presented in Figure S1.As illustrated in Figure S1, increasing the temperature from 298 K to 698 K results a significant rise in the current of SnSe post-O 3 adsorption within the low bias voltage range (0-0.6 V), markedly surpassing the current of SnSe without O 3 adsorption.Additionally, at 0.9-1.2V, the current of SnSe post-O 3 adsorption is distinctly higher than that of SnSe without O 3 adsorption.Figure 7 illustrates that SnSe exhibits a sensitivity to O 3 exceeding 200% at bias voltages of 0.2-0.6V and 0.9-1.2V.This observation suggests that, at elevated temperatures, the operational voltage range for O 3 detection by SnSe extends to the lower voltage range of 0.2-0.6V.
In addition to considering the influence of operating temperature, the selectivity of sensors towards the target gas in the presence of interfering gases is crucial for practical applications.To assess the selectivity of SnSe for O 3 detection, the I-V curves and sensitivities of SnSe upon exposure to O 3 and common air gases (CO 2 and O 2 ) are compared in Figures S2 and S3.As shown in Figure S2, the current of CO 2 -adsorbed SnSe within the bias voltage range of 0.1-1.2V is significantly lower than that of O 3 -adsorbed SnSe, indicating minimal interference from CO 2 in O 3 sensing.However, in the bias range of 0.1-0.7 V, the current of O 2 -adsorbed SnSe exceeds that of O 3 -adsorbed SnSe, leading to interference in O 3 detection.Therefore, 0.1-0.7 V is not an optimal bias range for SnSe-based O 3 sensing.At bias voltages greater than or equal to 0.8 V, the current of O 3 -adsorbed SnSe surpasses that of O 2 -adsorbed SnSe.The sensitivity curves in Figure S3 further demonstrate that within the bias range of 0.8-1.1 V, the sensitivity of O 2 -adsorbed SnSe remains below 90%, while the sensitivity of O 3 -adsorbed SnSe consistently exceeds 300%.At a bias voltage of 1.2 V, the sensitivity of O 2 -adsorbed SnSe increases to 262.15%, while the sensitivity for O 3 detection is approximately 6.9 times higher, reaching 1817.57%, and far exceeding that of O 2 -adsorption.These results confirm that SnSe exhibits high selectivity for O 3 detection in the presence of common air gases (O 2 and CO 2 ) within the bias range of 0.8-1.2V.
From the analysis presented, it is evident that within the bias voltage range of 0.9-1.2V, SnSe can detect O 3 across various temperatures without interference from high concentrations of ambient gases.Consequently, SnSe shows promise as a highly selective, highly sensitive, and reusable material for O 3 sensing.

Comparison
In this study, a comparative analysis of the SnSe monolayer with previously reported O 3 nanosensors is conducted, as detailed in Supplementary Table S1.In terms of electron transfer, the charge transfer amounts when O 3 interacts with stanene, B-doped stanene [12], BN fullerene-like nanocages [4], and Ni-decorated B 12 N 12 nanocages [3] are 0.557 e, 0.548 e, 0.5 e, and 0.789 e, respectively.These values are all significantly lower than the 0.881 e charge transfer value observed at the O 3 -SnSe interface.Compared to other two-dimensional materials such as MoS 2 [29] and SnS [14], SnSe exhibits an adsorption energy for O 3 that is 5.1 times and 1.5 times greater, respectively.Additionally, the electron transfer amount for SnSe is 7.0 times and 1.1 times higher than that of MoS 2 and SnS [14].Our findings reveal that the interaction between SnSe and O 3 is significantly stronger compared to other two-dimensional materials, while maintaining excellent reusability.
Notably, a greater chemical connection between the gas molecule and the surface is usually indicated by a larger charge transfer between SnSe and O 3 , which results in increased sensor sensitivity.Graphene oxide [11] has the highest documented sensitivity to O 3 for two-dimensional materials at 860%.This work, however, reveals that SnSe has a sensitivity of 1817.57% to O 3 , which is almost 2.11 times higher than that of graphene oxide.This suggests that SnSe has the potential to be a highly sensitive O 3 detector.

Conclusions
Our findings reveal that the SnSe monolayer exhibits remarkable potential as a highperformance O 3 sensing material.The calculated adsorption energy of −1.826 eV indicates a strong and spontaneous adsorption of O 3 molecules on the SnSe surface, suggesting efficient capture at room temperature.O 3 adsorption significantly alters the electronic structure of the SnSe monolayer, inducing a substantial band gap narrowing from 2.228 eV to 0.790 eV.

Figure 1 .
Figure 1.The structure and adsorption position of the SnSe monolayer: (a) Sn side; (b) Se side.

Figure 1 .
Figure 1.The structure and adsorption position of the SnSe monolayer: (a) Sn side; (b) Se side.As plotted in Figure2a, the O 3 molecule induces significant reconstruction of the SnSe monolayer surface during the adsorption process.Before adsorption, the Sn-Se bond length is approximately 2.744 Å, with a bond angle of 90.741 • .However, upon O 3 adsorption, the Sn atom beneath the adsorption site experiences a strong attractive force from the oxygen atoms, causing it to move closer to the oxygen atoms.This results in a contraction of the Sn-Se bond length to 2.657 Å, while the bond length between the Sn atom and the adjacent Se atoms elongates to 2.769 Å.Consequently, the Sn-Se-Se bond angle expands to 97.417 • , reflecting the substantial impact of O 3 adsorption on the SnSe surface structure.In the adsorption configuration, the distance between the closest oxygen atom of the O 3 molecule and the Sn surface is 2.251 Å, suggesting the possibility of a strong adsorption interaction between Sn and the O 3 molecule.

Figure 1 .
Figure 1.The structure and adsorption position of the SnSe monolayer: (a) Sn side; (b) Se side.

Figure 2 .
Figure 2. The most stable configuration (a) and differential charge density (b) of O 3 -adsopred SnSe monolayer.

Figure 7 .
Figure 7.The sensitivity of O3-adsorped SnSe monolayers at temperatures of 298 K and 698 K.

Figure 7 .
Figure 7.The sensitivity of O3-adsorped SnSe monolayers at temperatures of 298 K and 698 K.

Figure 7 .
Figure 7.The sensitivity of O 3 -adsorped SnSe monolayers at temperatures of 298 K and 698 K.